1. Field of the Invention
The invention pertains to a miniature water-proof video camera tube surveying system for the exploration of the interiors of bore holes or pipes and more particularly concerns such apparatus for investigating the geologic character of earth formations or the like penetratable by small diameter bores.
2. Description of the Prior Art
In the prior art bore hole surveying instruments, it appears to be customary to mount an image tube with a viewing face plate in a plane generally perpendicular to the bore hole axis in a tubular leak-proof envelope provided with a cylindrical viewing window forming a continuing part of the tubular leak-proof envelope. What part of the cylindrical bore hole surface is instantaneously viewed is determined by the position of a 45.degree. mirror collecting light from the field of view which is reflected by the plane mirror into an image-forming lens placed in front of the image tube viewing face. Optical scanning of the bore surface is accomplished by continuous or other rotation of the 45.degree. mirror generally about the bore hole axis and the viewed scene is presented at the earth's surface on the cathode ray tube screen of a raster scanned monitor display device.
A major defect of such prior art systems lies in the fact that the view presented by the monitor cathode ray tube of the bore hole surface rotates as the plane mirror rotates so that the operator finds it impossible to maintain knowledge of the orientation of the surface instantaneously under view. The operator never automatically knows which direction is up or down on the displayed image because the scan mirror is rotating relative to the monitor cathode ray tube screen and the upward direction therefore rotates on the monitor screen at the same rate as the scanning mirror.
A further problem relates to prior art arrangements for indicating magnetic north with respect to the scanned surface of the bore. In past arrangements, a pulse of light occurs when there is actual alignment between the rotating mirror and a north pointing part of a magnetic compass. With the help of a rotating 360.degree. scale mounted around the monitor cathode ray screen as a reference element, the magnetic direction of other parts of the viewed surface may be determined. This approach is complex and prone to error and measurements using it are not automatic and are, therefore, time consuming.
Prior devices suffer by yielding insufficient illumination of relatively low reflectance surfaces of the bore hole, especially in turbid water, since the required degree of simplicity and compactness has not been readily achieved with conventional light sources. Furthermore, prior art devices have used relatively high voltage power, transmitting it through the suspension cable, a practice found undesirable because heavier cables are required using extra thickness of electrical insulation.